Because of the speed diff?

Most externals are 5200RPM or 5400RPM while the bare drives are 7200RPM? There is also another way to save a royal ton of money, and that is refurbed drives. To makes sure it is good simply take a basic HDD test program (I personally use Spinrite on lvl II, this bypasses the firmware and does a basic read/write cycle on each sector and then reports the results) and if it passes you are good to go. I've bought several and using this test above the 2 that were actually bad were sent back and quickly replaced by good drives, the rest are still running happily, some after 5+ years.

That said your trick works fine if you don't mind a small boot speed hit. i personally am using a 5400RPM 1Tb Samsung as a boot drive and the benches show it 38% FASTER than the 400Gb Seagate it replaced. i'm just glad that right before this happened i bought 3Tb worth of Samsung drives to load in my desktop. I'll be able to ride this out just fine and dandy now!